Novel photoelectrocatalytic concepts for conversion of water, carbon dioxide, and nitrogen to fuels and chemicals

Today's route to fertilisers goes via ammonia (NH3) synthesized by the Haber-Bosch process, in which nitrogen (N2)and hydrogen (H2) react on a catalyst at high temperature and pressure, a very-energy demanding process driven by natural gas as hydrogen source, with huge CO2 emissions. In the PlusUltra project, we have explored the utilization of visible and ultraviolet light to split and bind nitrogen from the air to form ammonia (NH3) or nitrogen oxide (NO). The most challenging step remains the breaking of the strong triple N-N bond. We have studied different methods to activate N2 with help of energetic photons (UV light). The three partners NTNU, SINTEF and UiO have investigated different photocatalysts applied in different setups involving photocatalytic reactors and photoelectrochemical cells with aqueous or solid-state electrolytes. UV sources with increasing energy (shorter wavelength) towards the hard UV region have been installed in the reactors. Two setups have been used primarily for production of NH3 and one for NO, although it was increasingly realised that a combination within one setup producing ammonium and nitrite or nitrate ions is the most favourable in aqueous environments. A number of moderate bandgap oxide photocatalysts with or without doping have been tested for photocatalytic activation of N2 by soft UV-A irradiation in water. A range of high band gap nitride and oxide materials have been modeled theoretically and tested for various conditions and use of hard UV-C. The nitrides decompose under reaction conditions, but certain doped oxides have produced detectable amounts of ammonium salts. Several nanomaterials, such as metallic co-catalyst nanoparticles, and high band gap nanotubes have been fabricated and tested, but without significant success. In the last year of the project, the partners have actively engaged international experts in discussions on interpretation of results and are in the process of publishing a review paper, while interacting with national bodies and industry about follow-up projects on novel ideas that have emerged in the course of the project.

The biosphere converts sunlight to chemical energy by photosynthesis in plants, and mankind has learned to convert sunlight directly to electricity and electricity to light. Current nanoscience strives to convert sunlight and water/CO2 directly to fuels a nd in Graetzel cells to electricity via chemical redox cycles. In the future we need to make fuels and chemicals from CO2 and renewable energy and eventually to bind nitrogen and make fertilizers without natural gas and CO2 emissions. In PlusUltra we expl ore a number of novel concepts that address these challenges. We introduce the use of renewable and peak electricity in various forms, instead of or in addition to regular concentrated sunlight. We investigate novel photocatalysts based on recent advances in understanding defect chemistry, co-doping, nano- and heterostructures. For CO2 conversion we use parallel and sequential use of dual photocatalysts for UV and visual solar light in the form of composites and nanoarrays. We introduce solid electrolytes and mixed ion-electron conducting membranes as photoelectrode substrates for direct separation of products, furthermore allowing elevated temperatures that promote kinetics and increase resistance towards photo-corrosion. Finally, the above allow us to i ntroduce photolytic activation of nitrogen, driven by light and electricity and aided by novel photocatalysts. This subsequently reacts with supplied H2, O2, or C-containing molecules or their activated species. We hereby build a novel basis for using ren ewable energy to make fuels, carbon-containing chemicals, and fertilizers. The project lasts 3 years and employs postdoctoral researchers at UiO, NTNU, and SINTEF. The first phase focuses on novel photocatalysts, light sources, and characterization of the new processes. Evaluation after 1.5 years of the viability of the novel concepts forms basis for the second phase, in which focus is shifted to solid-state photoelectrochemical cells and optimization of processes.